A CSD system of the general type referred to herein for use in aircraft installation is shown in U.S. Pat. No. 3,733,924. The system disclosed therein employs a mechanical differential in conjunction with a hydrostatic transmission to rotate an aircraft alternator at constant speed via power supplied from a variable speed aircraft engine. The hydrostatic transmission is associated with a closed hydraulic circuit. Fluid is supplied to the circuit through a charge pump from a sump. Fluid pressure is regulated by a charge relief valve. An external circuit is provided for the IDG. This system does not include a valve which prioritizes the flow of oil to critical components during interruption of oil supply and which, at the same time, controls oil level in the sump.
Another aircraft CSD system is shown in U.S. Pat. No. 3,600,106. This system aims at reducing the required component weight in aircraft by providing a sump pressurization approach which eliminates the need for external engine bleed hardware. This aim is achieved by supplying air-entrained liquid into the transmission sump to attain a sufficient inlet head for the scavenge pump to maintain a desired efficiency level. A relief valve maintains a preset charge pressure in the fluid circuit. A differential pressure valve is used to maintain a sufficient pressure within a transmission sump so that there is sufficient pressure head at the inlet of a scavenge pump used for drawing leaking fluid from the transmission sump and delivering that fluid to the return conduit.
A charge pump is utilized to provide makeup,, lubricating and control fluid for the system. The charge or makeup fluid is fed through a conduit connected to a port in a hydraulic drive to make up for loss of fluid caused by leakage. Excess charge fluid flows over a charge relief valve which maintains a preset charge pressure in the conduit. A sump is provided in the case of the CSD to collect leakage fluid which is then pumped by means of a scavenge pump to a return conduit which passes through an external cooling circuit, i.e. the cooler is located outside the case or housing of the CSD system, and then to a reservoir where air entrained in the fluid is separated before the fluid is returned to the charge pump. However, no provision is made in this system for controlling oil sump level by diverting excess oil to an accumulator tank or setting priorities of oil supply between a number of functions sequentially.
Valves with movable spools have long been known for a variety of uses. For example, U.S. Pat. No. 4,537,284 discloses a progressive distributor valve for distributing lubricant to heddles of weaving machines. However, this valve is not designed for the purpose of controlling sump level or prioritizing flow to critical components.
Priority valves with movable spools in general are also known. Typically, these valves are designed to prioritize a function over other functions where there is an increased demand for fluid by the priority function. Conventional priority valves have not heretofore been used also as a device to control oil sump level. For instance, U.S. Pat. No. 4,556,078 shows a priority valve designed to control fluid flow between a power source and a hydraulic function in applications such as steering of agricultural and industrial off-road equipment. Pressurized fluid is supplied to one or more priority functions such as steering and to one or more non-priority functions such as hitches. However, the priority valve teaches prioritization of only one function and does not disclose how more than one function would be prioritized and in what order or sequence. Nor does it control sump oil level. This known priority valve operates by responding to an increased flow demand for providing a priority. It does not provide a mechanism for prioritization where an interruption in oil supply creates a drop in inlet pressure yet a need to supply oil to critical components for lubrication and cooling.
Another priority control for hydraulic steering is described in U.S. Pat. No. 4,665,939. Here, the pressure medium is divided between a priority "consumer" and a lower-ranking "consumer" via a priority valve in the form of a proportional valve. The flow of fluid to the consumers is regulated by pressure balance valves. If the priority consumer receives too little fluid, the proportional valve will act to throttle flow to the lower-ranking consumer. Again, it will be seen that this control prioritizes only one flow and does not control oil sump level. Moreover, priority of flow is related to a pressure decrease to the higher priority device.
U.S. Pat. No. 4,192,337 discloses another embodiment of a priority flow valve which operates in response to increased fluid demand by the single primary function, again typically a steering function. This system uses a valve with an additional land and port for conditional connection to an accumulator so that power stored in the accumulator can where desired be provided to the priority function.
The system described in U.S. Pat. No. 4,087,968 utilizes a solenoid-operated flow control valve and a pump in which displacement is varied to ensure that flow demand of the first work element and the plurality of second work elements is maintained. This flow control valve is not a priority valve since the flows are not prioritized between the first and second work elements.
The automatically shiftable direction control valve of U.S. Pat. No. 3,543,516 for a tractor hydraulic system which automatically directs the exhaust of continuously acting or double acting functions to a charging circuit so that there is always sufficient oil in the charging circuit to operate a priority function such as a system for supplying oil to cool and lubricate a tractor engine and transmission. The priority function is connected to the charging circuit which connects a charging pump output with a main pump input. If there is an increased flow demand, oil is recirculated through the main pump and bypasses the charging pump which is then free to supply oil only to the single primary function as opposed to the other functions.
Another conventional single priority function valve control system is described in U.S. Pat. No. 4,479,349. The distribution of fluid from a variable displacement pump to several different fluid-actuated devices is regulated by the control system. The priority valve gives fluid flow priority to the steering valve over the loader valve or the stabilizer valve of a combination backhoe and loader vehicle to ensure that increased flow demand in the steering valve will be given priority.
From the foregoing summary of typical priority valves of conventional construction and use, it can be seen that systems utilizing these valves have not been capable of prioritizing more than one function without depriving another function of fluid and are not designed to actuate the priority valve in response to pressure loss. Moreover, I am unaware that any conventional priority valve has heretofore been used to control sump oil level.